In the preparation of plastic protective mounts that are used to display stamps and other flat objects it is desirable to utilize a plastic which is of high strength, stability and possesses superb clarity. Such a plastic is a polyester film. For use as stamp mounts it has been found that films of 0.0010 to 0.0050" are statisfactory. There are various manufactures of polyester film (polyethylene terephthalate) which is nominally a thermoplastic film, but it is generally not regarded as heat sealable in an untreated state.
Many attempts have been made to marginally bond or seal polyester films without success. One known way that such sealing can occur is to manufacture a polyester film with a coextruded sealant. Various combinations of coatings (polyolefin, polymer, saran) on polyester film, on one or both sides, may be used to create a wide range of films with specific properties. However, such coatings would have the effect of reducing the clarity of polyester and introducing potentially detrimental coating in direct contact with the stamp. This could be detrimental when viewing a stamp for possible appraisal, sale or even just for the pleasure of a philatelist and for any long term storage, etc.
There are various prior art devices which have been used to form plastic envelopes such as U.S. Pat. No. 2,735,797. However, the patent is apparently aimed at polyethylene film and uses a bar sealer, which would not work on polyester. Additionally, the structure of U.S. Pat. No. 3,368,930 would not accomplish the intended result of the present invention. There, polyolefins are the subject of sealing with an air jet to cool the sealed area. This would be too late in the case of polyester.
Should untreated polyester films be attempted to be marginally sealed there is a disorientation of the film beyond the sealing area thus creating random or at best irregular seals which are not acceptable.